Flame treatment of a substrate

ABSTRACT

The present invention relates to a method and system for controlling flame treatment of a substrate.

TECHNICAL FIELD

The invention generally relates to the field of treatment of a substratesuch as a paper, paperboard or carton with a flame.

BACKGROUND OF THE INVENTION

The present invention relates to flame treatment of a substrate, such asa paper, paperboard or carton. The substrate is flame treated in orderto oxidize its surface to make it more polar. Combustion chemistryinvolves mixing a fuel and oxygen, for example propane and air andprovide ignition. The combustion reaction is in fact not only onereaction but many reactions. The flame contains many reactive species,such as hydroxyl radicals, carbonyl radicals, carboxyl radicals, oxygenatoms and ions etc. The reactive species are used to increase thesurface energy of the substrate and improve adhesion. The flame alsoheats and dries the substrate, which also can improve adhesion to aplastic material. There are many factors affecting the result, forexample the mixture of the fuel and air (oxygen). Conventionally theflame treatment device contains an analyzer controlling the mixture ofair and fuel. The mixture of air and fuel is important to obtain a flamehaving repeatable characteristics which is desired in order to obtain auniform treatment of the substrate. The substrate should be located on asuitable distance from the flame in order to obtain the desiredtreatment. FIG. 1 is a schematic view of a flame (2) emanating from aburner (1). Generally, in a simplified approach, the flame may bedefined as comprising two parts, an outer part (2) and an inner part(3). The temperature of the flame is not consistent throughout the flameand normally the edge of the inner part (3) is considered having thehighest temperature. In conventional flame treatment the substrate (5)should be arranged to obtain sufficient treatment, where sufficient iswhen positional relationship between the flame and the substrate remainswithin pre-determined limits (6). In order to obtain this positionalrelationship an operator visually observes the flame and arranges theburner in order to obtain the desired distance between the edge of theinner part (3) and the substrate. The adjustment is thus operatordependent and it is not normally easy to visually determine what part ofthe flame is suitable for placing the substrate. By visual adjustmentrepeatability is a problem. Additionally there are safety aspectsconcerning visual inspection of a flame.

There are drawbacks when arranging the substrate too close to the edgeof the inner part (3). There are also drawbacks in arranging thesubstrate to far from the part of the flame having the highesttemperature. In either case it is likely that sufficient treatment isnot obtained. Thus there is a need for methods and/or systemscontrolling the flame to obtain sufficient treatment of the substrate.

SUMMARY OF THE INVENTION

One object of the present invention to control the flame treatment of asubstrate. One object is to arrange a substrate suitable for flametreatment in in order to obtain sufficient treatment of the substrate.One object is to provide sufficient treatment in a continuos process,i.e. where the substrate is a continuously moving web-shaped material,such as a paper web, carton web or a paperboard web.

Objects of the present invention are achieved by a method for optimizingadhesion by controlling flame treatment of a substrate, in whichtreatment a flame emanating from a burner is applied to a substrate,said method comprising controlling the mass flow of fuel and air fed tothe burner, acquiring radical emission data by monitoring radicalemission emanating from the flame using a flame analyzer, processing theradical emission data, comparing the thus acquired radical emission datawith radical emission data from a database, and outputting the result ofthe comparison.

One object is achieved by the above mentioned method and wherein anerror signal is generate when the acquired radical emission data deviatefrom the radical emission data in the database.

One object is achieved by further comprising adjusting at least one ofthe mass flow of fuel and/or air fed to the burner, a relative anglebetween the burner and the substrate, and the distance between theburner and the substrate when the acquired radical emission data deviatefrom the radical emission data in the database.

One object of the present invention is achieved by a system forcontrolling flame treatment of a substrate, said system comprising aburner, a flame analyzer comprising a camera and a filter, wherein theflame analyzer is arranged to monitor radical emission emanating fromthe flame.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of thepresent invention, will be better understood through the followingillustrative and non-limiting detailed description of preferredembodiments of the present invention, with reference to the appendeddrawings, wherein:

FIG. 1 is a schematic illustration of a flame.

FIG. 2 is a schematic illustration of the system of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

There are many factors affecting the flame treatment of a substrate.Some important parameters are gas flow, gas type, fuel mixture (mixtureof fuel and oxygen, such as air), the distance between the burner andthe substrate, the angle between the burner and the substrate, and thespeed of the web substrate. Naturally it is preferred if many of theparameters are kept constant but normally this is not possible due tofactors such as clogging of part of the burner nozzle, affecting the gasflow; variations in the fuel mixture; and variation of the line speed(the line transporting the substrate). According to the presentinvention any fuel and mixture of different compatible fuels may beused, non-limiting examples thereof are natural gas, liquid petroleumgas (LPG) and methane. The fuel mixture is normally a mixture betweenone or more fuel(s) and oxygen. Air is often used as the oxygen sourceand the air-fuel ratio is often expressed as percent excess combustionair. As an example, excess combustion air of 15 percent means that 15percent more than the required stoichiometric air is being used.According to the present invention the excess oxygen is normally withinabout −5% to about +5%. The gas flow is generally within 5-50 m³. Otherparameters affecting the treatment of the substrate are the distancebetween the burner and the substrate; as well as the angle between theburner and the substrate. According to the present invention the anglebetween the burner and the substrate is normally about 90°. The anglemay however be anywhere within the interval 60-120°. As described aboveand in the accompanying claims the invention relates to a method forcontrolling flame treatment of a substrate, in which treatment a flameemanating from a burner is applied to a substrate, said methodcomprising controlling the mass flow of fuel and air fed to the burner,monitoring radical emission from the flame using a flame analyzer,wherein the positional relationship between a portion of the flame andthe substrate may be deduced.

Additionally the invention relates to a system for controlling flametreatment of a substrate, said system comprising a burner, a flameanalyzer comprising a camera and a filter, wherein the flame analyzer isarranged to monitor radical emission emanated by the flame.

According to the invention limits for the radical emission data isdetermined by measuring the radical emission from the flame (i.e.intensity) and comparing the emission with the treatment obtained, i.eif acceptable adhesion and/or surface tension is obtain. It is thuspossible to determine the lower and upper limit of the radical emissionwhere an acceptable substrate treatment is obtained. One way ofdetermining the radical emission limits is by a design of experiment(DoE) approach having surface tension and adhesion as substrateresponse. The experiment includes varying the gas flow, air/gas mixtureand the distance between the burner and the substrate. The experimentresults in determining upper and lower limits where acceptable treatmentof the substrate is obtain. The obtained limits are used to create adatabase which is used to compare the acquired radical emission datawith radical emission data from a database, and by measuring the radicalemission and comparing with the database acceptable substrate treatmentcan be obtained.

As described above the acquired radical emission data which for examplerelates to the intensity of the radicals at a position of the flame iscompared to values in a database and when an unacceptable deviation isobtained this is outputted in order for an operator to take action.Optionally an additional step adjusting at least one of the following:the mass flow of fuel and/or air fed to the burner, a relative anglebetween the burner and the substrate, and the distance between theburner and the substrate is per performed. Generally an unacceptabledeviation is a deviation of about more than 20% such as more than 10%,such as more than 5% such as more than 1%.

The radical emission is a measurement of the concentration of theradicals in the flame and the limits thus relates to concentrations ofradicals where acceptable treatment of the substrate is obtained. Whenthe radical is selected from hydroxyl radical the maximum hydroxylradical intensity occurs near the location of maximum flame temperature.Hydroxyl radicals are thus suitably used within the present inventionand as discussed below the concentration of hydroxyl radicals (OHradicals) are suitably measured at about 310 nm, where interference ofother radicals are within acceptable level. It is also possible toobtain an acceptance equation defining the shape and profile of theradical emission emanating from the flame. It has thus been discoveredthat the hydroxyl radical have an impact on the level of surfaceoxidation of the treated substrate. The radical emission may bemonitored over the whole flame or over selected portions of the flame.One example is to measure the radical emission intensity close to thesubstrate or even including the part where the flame hits the substrate.Monitoring also the part where the flame hits the substrate could bebeneficial as it will allow improved monitoring of the edges of thesubstrate. The flame analyzer may analyze only part of the obtainedinformation.

Additionally it is preferable but not a requirement of the presentinvention to avoid interfering light when monitoring the radicalemission. For this reason it may be beneficial to monitor radicalsemanating in the UV region (λ=10-400 nm) as conventionally there are noUV emission sources in the surface treatment device. One embodiment ofthe present invention relates to monitoring one or more, such as two,three or four radical emissions. For example the emission of hydroxylradicals could be combined with monitoring CN radical emission and/or CHradical emission. The system of the invention was used to imaging flamecharacteristics during testing of different settings of the flametreatment parameters. From these measurements, flame characteristicsresulting in surface tension and/or adhesion between the substrate and aplastic film or material extruded to the substrate was correlated to bewithin the specification set for surface tension or adhesion. Measuringthe emission of radicals made it possible to correlate what level ofradical emission that would lead to acceptable adhesion or surfacetesion between the substrate and the film or material extruded on thesubstrate. Based on the determination it was possible to set the limitsfor the radical emission. Adhesion between the substrate and the film ormaterial extruded on the substrate is normally determined by the tapetest or by using test equipment such a those marketed by Instron®. Theadhesion tests are those conventionally used and sufficient adhesion iswithin the capability of the skilled person to determine for example byusing the tape test. The measurement using the Instron® equipment isperformed according to ISO8510-2 using 180°, speed 50 mm/min and samplewidth of 15 mm. Sufficient treatment is considered to be above 10 N/m,such as above 30N/m.

The above mentioned radical emission is made by a flame analyzer, whichin one embodiment at least comprises a camera, for example a camerahaving at least one image sensor that detects light in the ultraviolet(UV) range, such as a charge coupled device (CCD) camera.

Suitable cameras are marketed as Princeton Instruments I-Max camera,SVS-Vistek camera, AVT-Stingray camera and AVT-Prosilca camera, equippedwith a CCD UV-sensor. The camera may if necessary also be equipped withone or more filter(s) allowing only light of certain wavelengths topass.

Emission from radicals typically resides in narrow wavelength regions,and thus it may be possible to select a specific radical by use of asuitable filter in front of the detector. Suitable filters for thispurpose are bandpass filters, transmitting emission in a narrowwavelength region. Bandpass filters are defined by their “centralwavelength”, corresponding to the centre of the transmission interval orthe wavelength of the maximum transmittance, and by their “bandpassregion” corresponding to the width of the transmission interval. Theproperty “Full Width at Half Maximum” (FWHM) is well established and mayalso be used for quantification of the bandpass region.

It is possible to measure the radical emission from many radicals butsome suitable examples are NH, OH, CN, C2 and CH radicals. The radicalsemit light at different wavelengths and if the emission from more thanone radicals are measured complementary information may be obtained. NHradicals emit in the range 330-340 nm and a suitable filter is AsahiXBPA340, having a maximum transmittance at about 340 nm and the fullFWHM is 10 nm; OH radicals emit in the range 281-343 nm and a suitablefilter is Asahi XBPA310, having a maximum transmittance at about 310 nmand the FWHM is 10 nm; CN radicals emit in the range 380-390 nm and asuitable filter is Asahi XBPA390, having a maximum transmittance atabout 390 nm and the FWHM is 10 nm; C2 radicals emit in the range470-564 nm and a suitable filter is Asahi XBPA520, having a maximumtransmittance at about 520 nm and the FWHM is 10 nm; CH radicals emit inthe range 390-440 nm and a suitable filter is Asahi XBPA430, having amaximum transmittance at about 430 nm and the FWHM is 10 nm. Othersuitable filters are marketed by Princeton Instruments.

In one embodiment it is the OH radicals that are controlled in themethod and system of the invention and thus the filter preferably hasλ_(max) at 310±5 nm, for example the Asahi XBPA310, having a maximumtransmittance at about 310 nm and the FWHM is 10 nm being suitably used.

The flame analyzer comprises the above mention camera and a filter and aprocessing unit. The processing unit analyzes the images obtained by thecamera and generates alarm and/or control signals. The camera may eithertake an image over the whole flame treatment area or the camera makes animage over one part of the flame treatment area followed by an image ofa subsequent part of the flame treatment area until images over thecomplete area are obtained. It is also possible to have the camerafocusing on one area of the flame treatment area (i.e. a specificportion of the flame) only but in certain aspects it is preferred to usethe complete flame treatment area (i.e. assemble the individual imagestake to get an image of the whole flame). The camera is preferablytaking many images per time unit, such as between 2-200 images duringthe period of 1-10 s.

The burner from where a flame emanates may be any suitable burner but itis preferred that the burner is of a dimension correlated to thesubstrate it is to treat, i.e. the width of the web substratecorresponds to the width of the flame. It is also possible according tothe present invention to use a burner emanating one or more flame(s).The flame may be of any type but typically flames considered as a longertype are used. It is however also possible to use flames of a shortertype, such as up to about 10 mm. It is also possible to use one or moreflames, originating from one or more burner(s), i.e. stacked flames. Incertain aspects stacked flames are beneficial as the treatment isperformed over a bigger area at the same time and using stacked flamesmay improve the treatment. In one embodiment the width of the flame isabout the same as the width of the web-shaped substrate that is beingtreated.

The processing unit may be used to evaluate many effects of the radicalemission; some examples are fluctuation of flame, i.e. pulsing overtime, evenness of flame, i.e. the flame consistency over the flametreatment area, flame shape and burner position. The processing unit isthus using image analysis to determine if the radical emission is withinthe limits. In one example of the invention the intensity distributionof one or more image(s) in the flame direction (from burner tosubstrate) is compared to a predefined curve. Thus enabling thepossibility to determine if the burner position in relation to thesubstrate is acceptable considering the current production settings ofcritical process parameters.

The flame analyzer may generate an alarm when one or more of thepre-determined criterias are outside the limits. The alarm may be anabsolute or relative alarm. An absolute alarm is when one or more of thepre-determined criterias are not fulfilled. A relative alarm is forexample when the intensity of the radical emission is reduced over timeand an alarm generated. Alternatively a relative alarm may be generatedwhen there are unacceptable differences over the treatment area. Thealarm may be generates as any suitable type of alarm.

In one embodiment the flame analyzer generates a signal instead of orcomplementary to the alarm. The signal is send to an appropriate devicethat is used to alter the criterias of the radical emission. Examples ofsuch devices are devices adjusting the fuel mixture and/or the rate ofgas flow rate. Other examples are devices adjusting the angle betweenthe burner and the substrate, preferably by tilting the burner; anddevices adjusting the distance between the burner and the substrate,preferably by adjusting the burner.

In one embodiment of the invention the flame analyzer generates a signaland optionally an alarm to a device adjusting the distance between theburner and the substrate. In one embodiment the flame analyzer generatesan alarm only and the distance between the burner and the substrate isadjusted manually. The adjustment, manual or automatic, should be suchthat an acceptable radical emission, determined by the flame analyzer,is obtained after the adjustment; or the adjustment procedure isrepeated until the radical emission is within the limits.

In one embodiment the flame analyzer further comprises an output devicefor displaying information about the system. Non-limiting examples ofinformation that may be displayed on the output device are images,alarms, control signals and the overall status of the system.

The substrate of the invention is in one embodiment paper, paperboard orcarton, for example a web-shaped paper, paperboard or carton.

In one embodiment the substrate is a plastic film, for example amulti-layered film or a stretched film such as an oriented film, e.g. amono- or biaxially oriented film, e.g. a BOPP film.

The invention as defined herein and in the accompanying claims generallyimproves the safety for the operator as the manual adjustment of thedistance between the burner and the substrate does not require visualinspection of the flame. The flame may thus be hidden for the operator.

Additionally the invention improves the process control treating asubstrate by a flame. The control is improved and thus adhesion betweensubstrate and a further material extruded onto the substrate may beimproved and optimized. The invention also provides a method forcontrolling the settings of flame treatment according to apre-determined specification, and facilitating the control thereof. Theinvention also ensure that performance settings are consistent betweendifferent manufacturing cites and thus improving the control and thespecification of the final material, wherever produced. The inventionenables sufficient adhesion over substantially the whole surface of thesubstrate.

According to one embodiment the present invention relates to treatmentof paper, paperboard or carton used in a lamination process, such as anextrusion lamination process, for preparing a material suitable forpreparing a package. Said package being used in food and dairy fillingequipment, such as aseptic packaging.

In one embodiment the present invention is used in the converting linewhere a paper, or paperboard or carton is laminated to a packaginglaminate used in packaging containers of the single use disposable typefor liquid foods. One such commonly occurring packaging container ismarketed under the trademark Tetra Brik Aseptic® and is principallyemployed for aseptic packaging of liquid foods such as milk, fruitjuices etc, sold for long term ambient storage.

The invention has mainly been described above with reference to a fewembodiments. However, as is readily appreciated by a person skilled inthe art, other embodiments than the ones disclosed above are equallypossible within the scope of the invention, as defined by the appendedpatent claims.

In FIG. 2 a schematic illustration of the system of the invention isdisclosed. The system discloses a burner (1) from which a flame (2)emanates. The flame hits a substrate (3) to be treated. The substrate(3), is arranged within a variable distance from the substrate (1). Thedistance (4) is obtained by monitoring radical emission from the flame(2) by a a flame analyzer which in FIG. 2 comprises a camera (5) havinga filter (6) and a processing unit (7), such as a computer, saidprocessing unit (7) may be integrated with the camera or a separateunit. The processing unit (7) use the information received by the flameanalyzer to determine if the radical emission is within pre-determinedlimits. The flame analyzer may also comprise an output device (8), forexample a monitor or a printer. The flame analyzer is arranged tomonitor (9) at least part of the flame. The limits may be used togenerate an alarm in order to discard substrate that has not beensufficiently treated. Additionally the distance (4) between thesubstrate (3) and the burner (1) may be adjusted, manually orautomatically until the radical emission is within the limits. Suchadjustment allows the substrate to be sufficiently treated.

1. A method for optimizing adhesion by controlling flame treatment of a substrate, said method comprising: controlling the mass flow of fuel and air fed to a burner; acquiring radical emission data by monitoring radical emission emanating from a flame produced from the burner, using a flame analyzer; processing the radical emission data; comparing the acquired radical emission data with radical emission data from a database; and outputting the result of the comparison.
 2. The method according to claim 1, wherein the flame analyzer utilizes a non-invasive monitoring technique.
 3. The method of claim 1, wherein outputting the result generates an error signal when the acquired radical emission data deviate from the radical emission data in the database.
 4. The method of claim 1, further comprising adjusting at least one of: the mass flow of fuel and/or air fed to the burner, a relative angle between the burner and the substrate, and the distance between the burner and the substrate. when the acquired radical emission data deviate from the radical emission data in the database.
 5. The method according to claim 1, wherein the substrate is paper, paperboard, carton or plastic film.
 6. The method according to claim 1, wherein the radical emission is hydroxyl radical emission, NH radical emission, CN radical emission, CH radical emission and/or C2 radical emission.
 7. The method according to claim 6, wherein the radical emission is hydroxyl radical emission.
 8. The method according to claim 1, wherein the flame analyzer comprises a camera and a filter.
 9. The method according to claim 8, wherein the camera is a camera having at least one image sensor that detects light in the ultraviolet (UV) range.
 10. The method according to claim 8, wherein the camera is a charge coupled device (CCD) camera.
 11. The method according to claim 8, wherein a filter having a λmax at 310±5 nm, 340±5 nm, 390±5 nm, 430±5 nm, or 520±5 nm is used.
 12. A system for controlling flame treatment of a substrate, said system comprising: a burner, and a flame analyzer comprising a camera and a filter, wherein the flame analyzer is arranged to acquire radical emission data from a flame.
 13. The system according to claim 12, wherein the system further comprises a processing unit to which radical emission data acquired by the flame analyzer is transferred and compared with radical emission data from a database.
 14. The system according to claim 12, wherein the processing unit that receives the data acquired by the flame analyzer is a unit integrated in the flame analyzer.
 15. The system according to claim 12, wherein the system further comprises an output device.
 16. The system according to claim 12, wherein the camera is a camera having at least one image sensor that detects light in the ultraviolet (UV) range.
 17. The system according to claim 12, wherein the camera is a charge coupled device (CCD) camera.
 18. The system according to claim 12, comprising a filter having a λmax at 310±5 nm, 340±5 nm, 390±5 nm, 430±5 nm, or 520±5 nm.
 19. The system according to claim 18, wherein the filter has a λmax at 310±5 nm. 